Carbon nanotubes (CNTs), comprising multiple concentric shells and termed multi-wall carbon nanotubes (MWNTs), were discovered by Iijima in 1991 [Iijima, S. Nature 1991, 354, 56]. Subsequent to this discovery, single-wall carbon nanotubes (SWNTs), comprising a single graphene rolled up on itself, were synthesized in an arc-discharge process using carbon electrodes doped with transition metals [Iijima, S.; Ichihashi, T. Nature 1993, 363, 603; and Bethune, D. S., Kiang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vasquez, J; Beyers, R. Nature 1993, 363, 605]. These carbon nanotubes (especially SWNTs) possess unique mechanical, electrical, and thermal properties, and such properties make them attractive for the next generation of composite materials. Carbon nanotubes are expected to serve as mechanical reinforcements for lightweight composite systems with further promise of multifunctionality. See Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. For instance, SWNTs possess a tensile strength of 50-100 GPa and a modulus of 1-2 TPa—five and ten times greater than steel, respectively, at just one sixth the weight. See Berber, S.; Kwon, Y. K.; Tomanek, D. Phys. Rev. Lett., 2000, 84, 4613; Lourie, O.; Wagner, H. D. J. Mat. Res. 1998, 13, 2418; Walters, D. A.; Ericson, L. M.; Casavant, M. J.; Liu, J.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Appl. Phys. Lett. 1999, 74, 3803; and Andrews, R.; Jacques, D.; Rao, A. M.; Rantell, T.; Derbyshire, F.; Chen, Y.; Chen, J.; Haddon, R. C. Appl. Phys. Lett. 1999, 75, 1329. However, the potential of using nanotubes as polymer composite reinforcements has, heretofore, not been realized, mainly because of the difficulties in processing and the limitation on load transfer. Several fundamental processing challenges must be overcome in order to fully enable the reinforcement by nanotubes. See Barrera, E. V. J. Mater., 2000, 52, 38. Due to the intrinsic van der Waals attraction the nanotubes have to each other, and by virtue of their high aspect ratio (e.g., ˜1:1000), nanotubes are typically held together as bundles and ropes, that have very low solubility in most solvents. See Ausman, K. D.; Piner, R.; Lourie, O.; Ruoff, R. S. J. Phys. Chem. B. 2000, 104, 8911; and Bahr, J. L.; Mickelson, E. T.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. Chem. Commun. 2001, 193. The dispersion property has become more important when nanotubes are blended with polymers. Nanotubes tend to remain as entangled agglomerates and homogeneous dispersion is not easily obtained. Furthermore, due to the atomically smooth non-reactive surface of nanotubes, lack of interfacial bonding limits load transfer from the matrix to nanotubes. In this situation, nanotubes are typically pulled from the matrix, rather than fractured, and play a limited reinforcement role. See Lourie, O.; Wagner, H. D. Appl. Phys. Lett. 1998, 73, 3527. Additional processing difficulties for nanotube reinforced epoxy polymer composites come from the significant increase of viscosity when the nanotubes are added directly into the epoxy.
A number of recent research efforts have used nanotubes for polymer composites reinforcement. See Geng, H.; Rosen, R.; Zheng, B.; Shimoda, H.; Fleming, L.; Liu, J.; Zhou, O. Adv. Mater. 2002, 14, 1387; Schadler, L. S.; Giannaris, S. C.; Ajayan, P. M. Appl. Phys. Lett. 1998, 73 (26) 3842; Ajayan, P.; Schadler, L.; Giannaries, C.; Rubio, A. Adv. Mater. 2000, 12, 750; Sandler, J.; Shaffer, M. S. P.; Prasse, T.; Bauhofer, W.; Schulte, K.;. Windle, A. H. Polymer 1999, 40, 5967; Vaccarini, L.; Desarmot, G.; Almairac, R.; Tahir, S.; Goze, C.; Bernier, P. AIP Conf. Proc. 2000, N. 544, 521; Gong, X.; Liu, J.; Baskaran, S.; Voise, R. D.; Young, J. S. Chem. Mater. 2000, 12, 1049; Spindler-Ranta, S.; Bakis, C. E. SAMPE 2002 Symposium & Exhibition, 2002; Biercuk, M. J.; Llaguno, M. C.; Radosavljevic, M. Appl. Phys. Lett. 2002, 80 (15). 2767; and Tiano, T. et al, Roylance, M.; Gassner, J. 32nd SAMPE Conf. 2000, p. 192. Some strategies have been proposed to overcome the various barriers to dispersion, including the use of ultrasonication, high shear mixing, surfactant addition, chemical modification through functionalization, wrapping the tubes with polymer chains, and various combinations of these. However, to date, only marginal success for nanotube reinforced epoxy composites has been realized, mainly because of the above-mentioned barrier to dispersion. Note that, dispersion has been more readily accomplished in thermoplastic polymer composites [Geng, H.; Rosen, R.; Zheng, B.; Shimoda, H.; Fleming, L.; Liu, J.; Zhou, O. Adv. Mater. 2002, 14, 1387], where stepwise dispersion was aided by high shear mixing, incipient wetting, and elongational flow.
Among polymer composites, high strength epoxy systems are very important materials, finding use in aerospace, electronics, and many other industrial applications. Consequently, carbon nanotube reinforced epoxy systems hold the promise of delivering superior composite materials with high strength, and lightweight and multifunctional features—if the problems of dispersal and integration can be overcome.
Purified multi-walled nanotubes (MWNTs) were first mixed and ultrasonically dispersed in epoxy resins by Ajayan and co-workers [Schadler, L. S.; Giannaris, S. C.; Ajayan, P. M. Appl. Phys. Lett. 1998, 73 (26) 3842]. The Raman spectroscopic response to tension and compression in cured epoxy composites, however, showed poor load transfer behavior, especially under tension. A later study, using single-walled nanotubes (SWNTs) at higher concentrations (e.g., 5 wt %) also showed that the nanotubes were slipping within the bundles and falling apart [Ajayan, P.; Schadler, L.; Giannaries, C.; Rubio, A. Adv. Mater. 2000, 12, 750]. Sandler et al. reported the difficulty in breaking up the entanglements of the nanotubes, although ultrasonication and the intense stirring process was found to improve the dispersion of the nanotubes [Sandler, J.; Shaffer, M. S. P.; Prasse, T.; 60
Bauhofer, W.; Schulte, K.;. Windle, A. H. Polymer 1999, 40, 5967]. Even on the millimeter scale, the distribution of nanotubes in such blends is not uniform within the epoxy. Vaccarini et al. [Vaccarini, L.; Desarmot, G.; Almairac, R.; Tahir, S.; Goze, C.; Bernier, P. AIP Conf. Proc. 2000, N. 544, 521] prepared several epoxy blends and composites with high concentrations (up to 35 wt %) of SWNTs. In this case, a linear increase of the Young's modulus with the weight percentage of the SWNTs was observed. These authors also pointed that the possible sliding of the SWNTs within the ropes and the bending of ropes limited any further mechanical enhancement since alignment was not produced. Biercuk et al. [Biercuk, M. J.; Llaguno, M. C.; Radosavljevic, M. Appl. Phys. Lett. 2002, 80 (15). 2767] reported a 125% thermal conductivity enhancement and a Vickers hardness increase by a factor of 3.5 when 2 wt % of SWNTs were added into epoxy.
Gong et al. [Gong, X.; Liu, J.; Baskaran, S.; Voise, R. D.; Young, J. S. Chem. Mater. 2000, 12, 1049] used surfactants as wetting agents to improve dispersion of nanotubes and observed an improvement in both the mechanical and thermal properties of the nanotube epoxy composites. Sean et al. [Spindler-Ranta, S.; Bakis, C. E. SAMPE 2002 Symposium & Exhibition, 2002] also prepared nanotube epoxy composites using a combination of surfactant addition and ultrasonic assistance for suspending the SWNTs in a large amount of acetone. However, no improvement of the modulus and the compressive strength for a filament wound composite with 1 wt % nanotube addition was observed. Microscopy revealed a non-uniform dispersion of nanotubes in the epoxy.
Despite the above-mentioned efforts, however, due to poor dispersion and weak interaction between pristine nanotubes and the surrounding matrix, the reinforcing role of high strength nanotubes in polymer composites is still quite limited. Chemical modification and functionalization have been shown to be feasible and effective means to improve solubility and dispersion of nanotubes. In addition, functionalized nanotubes can provide bonding sites to the polymer matrix so that the load can be transferred to the nanotubes to prevent separation between the polymer surfaces and nanotubes. See Calvert, P. Nature 1999, 399, 210. Theoretical calculations have predicted that even a high degree of sidewall functionalization will degrade the mechanical strength of SWNTs by only 15%. See Garg, A.; Sinnott, S. B. Chem. Phys. Lett. 1998, 295, 275.
A molecular simulation has suggested that the shear strength of a polymer-nanotube interface can be increased by over an order of magnitude with the introduction of even a relatively low density of chemical bonds between the single-walled nanotubes and matrix [S. J. V. Frankland, A. Caglar, D. W. Brenner, and M. Griebel, J. Phys. Chem. B. 2002, 106, 3046]. The calculation also predicted a negligible change in modulus for a (10,10) nanotube with the functionalization of at least up to 10% of the carbon atoms.
There exist numerous chemical routes for functionalization of nanotubes involving the covalent and/or non-covalent attachment of various functional groups to either nanotube end-caps or sidewalls. See Liu et al., Science 1998, 280, 1253; Chen et al., Science 1998, 282, 95; Bahr, J. L.; Tour, J. M. J. Mater. Chem. 2002, 12, 1952, Holzinger et al., Angew. Chem. Int. Ed. 2001, 40, 4002; Khabashesku et al., Acc. Chem. Res. 2002, 35, 1087.
The end-caps of SWNTs can be opened under oxidizing conditions and terminated with the oxygenated functionalities including carboxylic, carbonyl and hydroxyl groups [Liu et al., Science 1998, 280, 1253; Chen et al., Science 1998, 282, 95]. Oxidized nanotubes have better solubility and can form a well-dispersed electrostatically stabilized colloids in water and ethanol. See Shaffer, M. S. P.; Fan, X.; Windle, A. H. Carbon, 1998, 36(11), 1603. The presence of carboxylic acid functionalities offers opportunities for further derivatization reactions with a number of molecules. For example, oxidizing acid treated SWNTs can be further derivatized by reactions with thionyl chloride and long-chain amines [Hamon, M. A.; Chen, J.; Hu, H.; Chen, Y. S.; Itkis, M. E.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Adv. Mater. 1999, 11, 834; Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science, 1998, 282, 95; Chen, J.; Rao, A. M.; Lyuksyutov, S.; Itkis, M. E.; Hamon, M. A.; Hu, H.; Cohn, R. W.; Eklund, P. C.; Colbert, D. T.; Smalley, R. E.; Haddon, R. C. J. Phys. Chem. B 2001, 105, 2525] or by esterification [Riggs, J. E.; Guo, Z.; Carroll, D. L.; Sun, Y.-P. J. Am. Chem. Soc. 2000, 122, 5879; Sun, Y.-P.; Huang, W.; Lin, Y.; Fu, K.; Kitaigorodsky, A.; Riddle, L. A.; Yu, Y. J.; Carroll, D. L. Chem. Mater. 2001, 13, 2864].
Sidewall functionalization of CNTs, like end-cap functionalization, offer opportunity, if the right functional moiety is attached, to covalently integrate into epoxy polymer matrices, but they offer far more opportunities for such integration by virtue of having more functional groups with which to interact.
The use of functionalized nanotubes for epoxy composite fabrication has been reported by Tiano et al. See Tiano, T. et al, Roylance, M.; Gassner, J. 32nd SAMPE Conf. 2000, p. 192. Here, the sidewall surfaces of the nanotubes were ostensibly functionalized via free-radical polymerization of poly (methyl methacrylate) using AIBN as a catalyst. It was presumed that the CNTs would form free radical weak spots that would then react with the methyl methacrylate monomers. These “functionalized” CNTs were then mixed into an epoxy resin and allowed to cure. With a 1 wt % load of functionalized nanotubes in the epoxy, a significant improvement in the mechanical properties was observed: an 11% increase in stress and a 21% increase in modulus over the unfilled epoxy was demonstrated, which differs markedly from the observed sharp decrease of these parameters when using pristine nanotubes.
As a result of the foregoing, it should be understood that methods for exploiting end-cap and/or sidewall functionalized carbon nanotubes to realize better dispersion in, and/or better covalent bonding with, epoxy matrices will significantly advance the integration of carbon nanotubes into epoxy polymer composites and subsequently provide enhancement in the properties of such composites, allowing nanotube-epoxy systems to realize their full potential.